I would have to say Firefox with the web developer's toolbar installed, that way you can edit CSS and immediately see how it affects your website.
try this: http://www.causeway.co.uk/freestuf/cssedit/download.htm i am using it and it is very useful
I'm pretty confortable with Dreamweaver for CSS editting. Auto-complete for fast coding and basic colouring to help correct any typo errors.
I'm with Clive, DW works really well when I'm coding CSS. For testing purposes though, I use the Web Developer extension for Firefox. (http://chrispederick.com/work/webdeveloper/)
You're asking the wrong question. CSS is simply text. Any text editor will work just fine. Any. The question you should ask is, "what features would I find helpful when authoring css?" Take the info you get, figure out which features might be important to you, and pick from the editors (probably most) that provide them. For myself, I don't enable any special features for css. :shrug: cheers, gary
I use UEStudio for editing of all my scripts and files (PHP, PERL, CSS, ActionScript, HTML and other) http://www.ultraedit.com/
Dreamweaver has a bunch of extra "fluff" to it which may or may not be handy depending on what you are doing. DW is strictly commercial. TopStyle is another good app but who the hell wants to shell out 80 bucks? I recommend NotePad++ for a good free text editor. It has great functionality and supports all major scripting languages such as CSS, PHP, HTML, C, etc...
Yeah, if you just want a basic text editor, Notepad++ is great. Also, if you're on Mac, try TextWrangler.
I'd say go for ones with the huge time saving editors that do code completion like dreamweaver; I know Aptana does as well, but it's HTML editing abilities aren't great. It needs a good find and replace feature.